384X series PWM controller is an industrial standard IC for power supply control, and it is widely used in AC-to-DC switching power supply for electronic products such as computes, TVs, DVDs.
The household appliances and office equipments used by people in life and work every day generally employ switching power supplies. The light load and standby mode functions are a big issue and have received a lot of attention from government institutions. In U.S.A. there is a government decree in Jul. 2001 ordering electric equipment procured by government institutions ought to have standby power loss less than 1 W. The environmental protection organizations in other countries also set up similar plans and standards. For instance, the “Energy-star” of U.S.A and “Blue Angel” of Germany are standards adopted by a growing number of areas.
Chinese government also takes a serious look on the energy conservation issue. Energy saving procurement is a policy actively implemented. And China Energy Saving Product Authentication Center also embarks a series of research projects. The first one is the energy conservation authentication project for standby power consumption of color TV. It plans to target two to three products every year for energy conservation authentication, including video products such as DVD, VCD and business machines such as printer, facsimile machine, computer and the like. Hence how to reduce the standby power loss of the product has become an important research subject for most power supply manufacturers. The research of chip manufacturing for reducing power loss under light loads and standby mode has some progresses. A few known semiconductor manufacturing companies have introduced green low power consumption power supply control chips, such as NCP120X series of Onsemi, FAN7601 of Fairchild, ICE2AS01 of Infineon, TEA1533 of Philips, SG684X series of System General, and the like.
Most power supply users now use 384X controllers. They urgently need a new chip that can save energy to replace the 384X chip. However, the chips mentioned above do not equip such a capability.
FIG. 5 illustrates the block diagram of a conventional 384X controller 500. The automatic circuit 102 has a power supply input 7 to receive the input of an external power supply and a ground end 5 to provide electric power to a voltage reference circuit 104. The voltage reference circuit 104 provides an accurate reference voltage for a logic control circuit 106 and an error amplifier 112, and outputs the reference voltage to an external circuit through a reference voltage output end 8. The logic control circuit 106 receives the reference voltage to generate a logic control signal.
There is an oscillator 4 with a control input end 4 which receives a control signal from the external circuit. A pulse signal of a constant frequency is generated based on the control signal. The pulse signal is provided to an output circuit 110. The control input end 4 is connected to a resistor and a capacitor (shown by 409 and 410 in FIG. 4) to alter the constant frequency and the maximum duty cycle of the pulse signal. The output circuit 110 has a power supply input end 7, a ground end 5 and a signal output end 6. The power supply input end 7 receives an output of the external power supply, and generates an output signal based on the logic control signal generated by the logic control circuit 106 and the pulse signal generated by the oscillator 108 to provide the external circuit through the signal output end 6.
The error amplifier 112 has a compensated output end 1 and a voltage feedback input end 2. When in use, the ports of 1 and 2 are bridged by an external resistor and a capacitor in a parallel manner (shown by 405 and 406 in FIG. 4) to compensate gain and frequency. The error amplifier 112 generates an amplified error signal based on the reference voltage of the voltage reference circuit 104 and the voltage feedback signal output by the external circuit received by the voltage feedback input end 2. A comparator 114 provides a current sense input 3 to compare a current sense signal voltage fed to the current sense input 3 by the external circuit and the voltage of the amplified error signal to generate a comparison signal which is provided to a PWM latch 116. The PWM latch 116 corresponds to the comparison signal and the pulse signal transferred from the oscillator 108 to generate a pulse control signal. The potential of the pulse control signal determines the output circuit 110 on and off.
During operation, when the system starts in a normal condition and carries a load, the signal voltage at the current sense input 3 of the comparator 114 rises gradually from zero. The input signal voltage at another end of comparator 114 is provided by the error amplifier 112 according to the voltage feedback signal output by the external circuit. The error signal voltage of the error amplifier 112 is greater than the voltage of current sense signal. The comparator 114 generates a comparison signal based on the comparison result and provides to the PWM latch 116. The PWM latch 116 may be a RS trigger with a control end to receive the pulse signal of the oscillator 108 and another control end to receive the comparison signal of the comparator 114. When the pulse of the oscillator 108 is activated, the PWM latch 116 generates a pulse control signal based on the comparison signal to turn on the output circuit 110. Thereby the output circuit 110 outputs the pulse signal from the oscillator 108 to the external circuit. When the voltage of the current sense signal rises and is greater than the voltage of the error signal, the comparator 114 reverses, the pulse control signal condition generated by the PWM latch 116 also changes, such as from a high potential to a low potential, to turn off the output circuit 110. The current sense signal becomes zero. As the voltage feedback signal output by the external circuit still exists, the error signal of the error amplifier 112 also exists. Meanwhile, the voltage of the current sense signal is smaller than the voltage of the error signal. The comparator 114 is reversed again. When the pulse of the oscillator 108 reaches the PWM latch 116, the output circuit 110 turns on again to complete an on-and-off cycle. A number of on-and-off cycles are repeated. As the time of turning off the output circuit 110 is very short, the waveform of the output signal forms a continuous pulse wave. Such an operation fashion is called the PWM mode.
When the system is in a light load or a standby condition, the error signal generated by the error amplifier 112 is smaller when the load is carried, then the time of the voltage of the current senseing signal risen and greater than the voltage of the error signal is shortened, PWM duty cycle is also shortened. The output waveform is still a continuous pulse waveform same as in the PWM mode at a smaller duty cycle with the frequency same as the oscillator 108. Energy consumption is no big different from the normal operation condition. The present invention aims to provide an energy-saving controller on this basis.